The IP Multimedia Core Network Subsystem (IMS) is an IP-based network architecture proposed by the 3rd Generation Partnership Project (3GPP) organization, which constructs an open and flexible service environment, supports multimedia applications, and provides rich multimedia services for users.
At present, the mobile communication is mainly dominated by Circuit Switched (CS) networks, such as the Global System for Mobile Communications (GSM) and the Code Division Multiple Access (CDMA) and so on, and there have been specifications and patents with respect to the User Equipment (UE) accessing the IMS via the CS currently.
In the specifications such as the TS 24.292, the 3GPP TS 23.237 and the 3GPP TS 24.237 etc., there have been signaling and flow descriptions for implementing an emergency call transfer between Packet Switched (PS) and CS by using the EATF entity.
As shown in FIG. 1, it is a flow diagram of signaling of the existing emergency call PS-CS transfer process, and the process includes the following steps.
In step 101, a user initiates an emergency call via the PS access.
In step 102, a Proxy-Call Session Control Function (P-CSCF) routes the emergency call to an Emergency-Call Session Control Function (E-CSCF) according to the existing mechanism.
In step 103, the E-CSCF routes the call to an EATF entity.
In step 104-105, the EATF entity anchors the call, and sends the call to the original E-CSCF.
In step 106-111, the follow-up emergency call flow is completed according to the existing mechanism.
In step 112, the user moves to the CS, initiates an emergency call transfer invite to an IMS network via an enhanced Mobile Switching Center (eMSC) server, and routes the invite to an Inquiry-CSCF (I-CSCF) of a home domain.
In step 113, the I-CSCF routes the transfer invite to the EATF entity according to the existing mechanism.
In step 114, the EATF entity identifies that the received invite (INVITE) message which requests for performing transfer of a certain existing emergency call, and initiates a media negotiation flow to an existing session of the other side.
In step 115-117, the media negotiation is implemented according to the existing mechanism.
In step 118-119, response message is sent to the eMSC server, the acquired far-end media is included in the message, and a media channel between the user (accessing via the CS) and a Public Safety Answering Point (PSAP) is established to implement a continued call.
In step 120-125, the EATF entity initiates to release an original PS access side session.
The follow-up flow is identical with the existing relevant specifications, which will not be described in detail here.
However, the related art has the following problems: the role and functions of the EATF entity are similar to those of a Service Centralization and Continuity Application Server (SCC AS) in the ordinary call transfer, and the problem of long transfer process exposed by the Single Radio Voice Call Continuity (SR VCC) transfer will exist.